Display walls are used in different application areas, such as indoor and outdoor advertising, for events such as concerts, sport games or general entertainment. Display walls are available in different sizes up to ten or more square meters. Different content, such as video, text or graphics can be shown on display walls, both as static or moving images. Display walls are usually built by positioning display units next to each other.
For example, a display unit can be a liquid crystal display or a LED module. Examples of display walls comprising large numbers of LED modules are given in e.g. EP 1 238 328 B1 “Method of and device for displaying images on a display device”, US2005/0134525 “Control system for a tiled large-screen emissive display” and US2009/0312884 “Method for power sharing and controlling the status of a display wall”.
The display unit can also be a rear projection display. Example of display walls comprising a large number of rear projection display modules are given in e.g. U.S. Pat. No. 6,335,829 “Projection screen for image reproduction devices which are positioned next to and/or above one another”. In this case, the display wall is a rear projection screen assembled from a plurality of rear projection screen panels. A supporting structure supports the screen panels and the projectors. In some instances, there may be one projector for more than one screen panels or one projector per screen panel. In any case, a large video wall will comprise at least 2 projectors.
Assembly and maintenance of large video walls both benefit from modularity. In a modular display walls, the display units are identical. In particular, each display unit has its own DC power source to power not only an image forming device; whether it is a projector, a LED panel or a liquid crystal display; but also among other things processing means to receive, process and/or send control and data signals and “housekeeping” electronics to, among other things, be able to turn on the display wall when it is in standby upon receipt of an instruction issued by a central command unit (e.g. a computer).
Since each display tile is preferably supplied with its own DC power supply, an alternating current power supply such as a 120 volt AC or 240 volt AC power distribution system can be provided for the display wall. Using an efficient switching DC power supply, each display tile will require more or less between 0.8 and 1 ampere of AC current, sometimes more. Using an efficient switching DC power supply, each display can require up to 400 W, for example depending upon the video content, hence the maximum current at low input AC voltage (120 V or less) can reach as much as 6 A. The DC power supply is often not 100% efficient and can dissipate power even when the image forming device is in standby mode. For example, a conventional HD Liquid Crystal (LC) Display can dissipate around 0.5 to 1 W. Assuming a display wall that includes one hundred twenty-eight display tiles, the current drawn in a stationary state in stand-by mode would be in the order of 64 to 128 W.
For large video walls, the power dissipation in standby mode becomes important enough to prevent the display wall from complying with “green” labels or eco-labels like e.g. EPEAT, Group for Energy Efficient Appliances Label, Energy Star . . .
When images are displayed, the typical current needed by e.g. a LED or LC display unit is 1 A but it can go up to 6 A depending upon the input AC voltage, video content and the type of display (LCD or Rear Projection). If all the tiles are switched on at the same time, the inrush current would be very large and the AC power supplies for the video wall display would have to be dimensioned accordingly. Large current fluctuations on the AC power supply line would also generate significant electromagnetic radiation that could interfere with e.g. digital devices, mobile phones etc . . .